Osmotic regulation of transcription in Lactococcus lactis: ionic strength-dependent binding of the BusR repressor to the busA promoter.

The busA locus of Lactococcus lactis encodes a glycine betaine uptake system. At low osmolarity, the transcription of busA is repressed by the BusR protein, which is responsible for the osmotic inducibility of the busA promoter (busAp). In this work, we investigated the mechanism of the osmo-dependent repression by BusR. We found that BusR binding to the busA promoter is dependent on the ionic strength in vitro. Using a BusR derivative carrying a phosphorylation site and the Escherichia coli RNA polymerase holoenzyme, we showed that these proteins are able to form a stable ternary complex by both binding to the same busAp fragment. The association/dissociation of BusR to the RNA polymerase-busAp complex is strictly correlated to the surrounding ionic strength. Together, these results suggest that during growth at low osmolarity BusR represses transcription from busAp at a step further the recruitment of the RNA polymerase. At high osmolarity, an elevated cytoplasmic ionic strength would dissociate BusR from busAp, resulting in the osmotic induction of the busA operon.